PROJECT SUMMARY/ABSTRACT Electronic nicotine delivery systems (ENDS) are marketed as ?safer? products and increasingly used in the U.S. and worldwide. Considering that ENDS are highly engineered products containing plastic, glass and metal parts as well as e-liquids, toxic compounds that are not typical hazards for users of combustible tobacco products may exist in ENDS. Recent studies suggesting that ENDS aerosol contains flame retardants, and that ENDS users had significantly higher urinary metabolites of tris(2-chloroethyl) phosphate (TCEP), a flame retardant, than nonusers, focus attention on these potentially novel risks. The premise of this research is that components of ENDS devices and ingredients in refill solutions are manufactured using materials that may contain ECCs, including phthalate plasticizers, phenolic compounds, and flame retardants. These chemicals have been widely applied as plasticizers, flame retardants, lubricants, anti-foaming stabilizers, and surfactants in plastics, electronics, medical devices, and other consumer products. Because these chemicals are not bonded to the materials in which they present, they can leach into the surrounding media with time. Studies have documented that exposure to ECCs resulting from consumables are associated with adverse health outcomes, including carcinogenic activity, endocrine disruption, reproductive and developmental abnormalities. However, information regarding the types and levels of ECCs in ENDS is still lacking, and current knowledge regarding the safety of ENDS may underestimate the health risks from ECCs. In particular, ECCs in ENDS may be a prominent concern, given the observed general lower levels of known toxics in ENDS aerosols compared to cigarette smoke. Concurrent dearth of the scientific data and specific regulatory rules necessitates an investigation of the types and levels of ECCs in ENDS. Our specific aims are: (1) characterize the contamination of e-liquids with ECCs by identifying the types and levels of ECCs in e-liquids; (2) identify and measure the types and levels of ECCs in certain parts of ENDS, including refillable cartridge/tanks, as well as mouthpieces, which are potential exposure sources of ECCs; (3) characterize the types and levels of ECCs in ENDS aerosols, thus to provide vital information in future efforts to assess the risks from ECCs attributable to ENDS use; (4) examine and characterize the similarities and differences in types and levels of ECCs in e- liquids, extracted samples, and ENDS aerosols. Filling this knowledge gap is fundamental to provide data that directly informs and contributes to evidence-based safety evaluation of ENDS products, and to development of regulatory product standards for ECCs in ENDS. Additionally, the results from this exploratory research can be used as a point of reference for future work examining trends in ECCs in ENDS as the market continues to evolve, new product designs are introduced, and FDA regulation of the market increases.